High voltage AC machine winding with grounded neutral circuit

ABSTRACT

An electric high voltage AC machine intended to be directly connected to a distribution or transmission network ( 16 ) comprises at least one winding. This winding comprises at least one current-carrying conductor, a first layer having semiconducting properties provided around said conductor, a solid insulating layer provided around said first layer, and a second layer having semiconducting properties provided around said insulating layer. In addition grounding means ( 18, 24, 26, 28 ) are provided to connect at least one point of said winding to ground.

The present invention relates to an electric high voltage AC machineintended to be directly connected to a distribution or transmissionnetwork, said machine comprising at least one winding.

Such generators with a rated voltage of up to 36 kV is described by PaulR. Siedler, “36 kV Generators Arise from Insulation Research”,Electrical World, Oct. 15, 1932, pp. 524-527. These generators comprisewindings formed of medium voltage insulated conductors whereininsulation is subdivided into various layers of different dielectricconstants. The insulating material used is formed of variouscombinations of the three components of micafolium-mica, varnish andpaper.

In a publication by Power Research Institute, EPRI, EL-3391, Apr. 1984 agenerator concept is proposed for providing such high voltages that thegenerator can be directly connected to a power network without anyintermediate transformer. Such a generator was supposed to comprise asuperconducting rotor. The magnetization capacity of the superconductingfield would then make it possible to use air gap windings of sufficientthickness for withstanding the electric forces. The proposed rotor is,however, of a complicated structure with a very thick insulation whichconsiderably increases the size of the machine. In addition theretospecial measures have to be taken for insulating and cooling the coilend sections.

By electric high voltage AC machines is meant, according to the presentinvention, rotating electric machines like generators in power stationsfor production of electric power, double-fed machines, outer polemachines, synchronous machines, asynchronous converter cascades, as wellas power transformers. For connecting such machines, except fortransformers, to distribution and transmission networks, in thefollowing commonly referred to as power networks, a transformer has sofar been needed for transforming the voltage up to the network level,that is in the range of 130-400 kV.

By manufacturing the winding of these machines of an insulated electrichigh voltage conductor with a solid insulation of similar structure ascables used for power transmission the voltage of the machine can beincreased to such levels that the machines can be directly connected toany power network without an intermediate transformer. Thus thistransformer can be omitted. Typical working range for these machines is30-800 kV.

For this kind of machines special attention has to be paid to groundingproblems.

Grounding of generator systems and other similar electrical systemsimplies intentional measures for connecting an electric system to groundpotential. When the so-called neutral point of the system is availableit is often connected to ground, directly or through a suitableimpedance. It also happens that other points in the system are connectedto ground. If one point in the system is grounded the complete system isgrounded as long as the galvanic connection extends.

The grounding principle used is determined by the design of the system.For a system including a generator directly connected to a Y-Δ connectedstep-up-transformer with the Δ-winding at the generator voltage thefollowing grounding alternatives are most common.

High resistance grounding

No grounding

Resonant grounding.

High resistance grounding is normally realized by connection of a lowohmic resistor in the secondary of a distribution transformer with theprimary winding of the transformer connected from the generator neutralpoint to ground. Such prior art grounding is illustrated in FIG. 1,which shows a generator 2 connected by a Y-Δ connected step-uptransformer 3 to a network 9. The primary 11 of a distributiontransformer is connected between the neutral point of the generator 2and ground. In the secondary 10 of the transformer a resistor 12 isconnected.

The same kind of grounding can, of course, be obtained by installing ahigh ohmic resistor directly between the generator neutral point andground.

An ungrounded electric system lacks any intentional connection toground. Thus an ungrounded generator has no connection between itsneutral point and ground, except for possible voltage transformers forfeeding different relays and instruments.

Resonant grounding is normally also realized as illustrated in FIG. 1with the resistor 12 replaced by a reactor 12 a. The reactor reactanceis chosen such that the capacitive current during a line to ground faultis neutralized by an equal component of inductive current contributedfor by the reactor 12 a.

Also low resistance or low impedance grounding and effective groundingof the above systems are possible. Low resistance or low impedancegrounding will result in lower transient overvoltages but higher groundfault currents, which can cause internal damages to the machine.

Low resistance grounding is achieved by the intentional insertion of aresistance between the generator neutral and ground. The resistance maybe inserted either directly in connection to ground or indirectly, inthe secondary of a transformer whose primary is connected betweengenerator neutral and ground, cf. FIG. 1.

Low impedance grounding, that is low inductance grounding isaccomplished in the same way as low resistance grounding with thesubstitution of an inductor for the resistor. The value of the inductorin ohms is less than that required for resonant grounding, as discussedabove.

For systems comprising several generators connected to a common feedingline or bus with circuit breakers between the generator terminals andthe common bus low resistance or low impedance grounding is suitable.

Effectively grounding the neutral of a generator has substantially thesame advantages and disadvantages as the low resistance or low impedancegrounding with some differences.

A system is said to be effectively grounded if certain impedancerequirements, which restricts the size of the grounding impedance, arefulfilled. In an effectively grounded system the maximum phase-to-groundvoltage in unfaulted phases, in case of a ground fault, are limited to80% of phase-to-phase voltage.

A power system network is mainly grounded through ground connections ofneutral points of transformers in the system and can include noimpedance (except for contact resistances), so-called direct grounding,or have a certain impedance.

Previously known grounding techniques are described in e.g. thepublication IEEE C62.92-1989, IEEE Guide for the Application of NeutralGrounding in Electrical Utility Systems, Part II—Grounding ofSynchronous Systems, published by the Institute of Electrical andElectronics Engineers, New York, USA, Sep. 1989.

If the generator neutral is grounded through a low resistance orinductance as discussed above, a path is formed for third harmoniccurrents from the generator neutral to ground. If a directly grounded orlow-impedance grounded transformer winding or another low-impedancegrounded generator is directly connected to the generator, the thirdharmonic currents will circulate therebetween under normal conditions.

Techniques for solving the problems of third harmonic currents ingenerator-and motor-operation of AC electric machines of the kind towhich the present invention relates are described in Sweedish patentapplication Ser. Nos. 9602078-9 and 97003-9.

The purpose of the present invention is to provide an electric highvoltage AC machine suitable for direct connection to distribution ortransmission networks as indicated above, which machine is provided withgrounding means suitable for different uses and operating conditions ofthe machine.

This purpose is obtained with an electric high voltage AC machine of thekind defined in the introductory portion of the description and havingthe characterising features of at least one winding comprising at leastone current-carrying conductor and a magnetically permeable, electricfield confining covering surrounding the conductor; a first layer havingsemi-conducting properties surrounding the conductor, a solid insulatinglayer surrounding said first layer, and an outer layer havingsemi-conducting properties surrounding said insulating layer, andgrounding means for connecting the neutral paint of said winding incircuit to ground.

An important advantage of the machine according to the invention residesin the fact that the electric field is nearly equal to zero in the endregion of the windings outside the second layer with semiconductingproperties. Thus no electric fields need to be controlled outside thewinding and no field concentrations can be formed, neither within thesheet, nor in winding end regions, nor in transitions therebetween.

According to an advantageous embodiment of the machine according to theinvention at least two adjacent layers have substantially equal thermalexpansion coefficients. In this way defects, cracks or the like as aresult of thermal motions in the winding, are avoided.

According to another advantageous embodiment of the machine according tothe invention said grounding means comprise means for low resistancegrounding of the winding. In this way transient overvoltages as well asthe ground fault current can be limited to moderate values.

According to still another advantageous embodiment of the machineaccording to the invention, wherein the machine has a Y-connectedwinding, the neutral point of which being available, high resistancegrounding means comprise a resistor connected in the secondary of atransformer whose primary is connected between the neutral point andground. In this way the resistor used in the secondary of thetransformer is of comparatively low ohmic value and of ruggedconstruction. Sufficient damping to reduce transient overvoltages tosafe levels can be achieved with a properly sized resistor. Further,mechanical stresses and fault damages are limited during line-to-groundfaults by the restriction of the fault current. Such a grounding deviceis also more economical than direct insertion of a high ohmic resistorbetween the generator neutral and ground.

According to another advantageous embodiment of the machine according tothe invention, wherein the machine has a Y-connected winding the neutralpoint of which being available, the grounding means comprises a reactorconnected in the secondary of a transformer whose primary is connectedbetween the neutral point and ground, said reactor havingcharacteristics such that the capacitive current during a ground faultis substantially neutralized by an equal component of inductive currentcontributed for by the reactor. In this way the net fault current isreduced to a low value by the parallel resonant circuit thus formed, andthe current is essentially in phase with the fault voltage. The voltagerecovery on the faulted phase is very slow in this case and accordinglyany ground fault of a transient nature will automatically beextinguished in a resonant grounded system.

According to still other advantageous embodiments of the machineaccording to the invention the grounding means comprise a Y-Δ groundingtransformer or a so-called zigzag grounding transformer connected to thenetwork side of the machine. The use of such grounding transformers areequivalent to low inductance or low resistance grounding with respect tofault current levels and transient overvoltages.

To explain the invention in more detail embodiments of the machineaccording to the invention, chosen as examples, will now be describedmore in detail with reference to FIG. 2-11 on the accompanying drawingson which

FIG. 1 illustrates prior art grounding of a synchronous generator,

FIG. 2 shows an example of the insulated conductor used in the windingsof the machine according to the invention,

FIG. 3 shows an ungrounded three-phase machine in the form of aY-connected generator or motor connected to a power system,

FIGS. 4-13 show different examples of grounding the Y-connected machinein FIG. 3,

FIG. 14 shows a machine according to the invention in the form of aΔ-connected generator or motor connected to a power system, and

FIG. 15 illustrates the use of a grounding transformer in the systemshown in FIG. 14.

In FIG. 2 an example is shown of an insulated conductor, which can beused in the windings of the machine according to the invention. Such aninsulated conductor comprises at least one conductor 4 composed of anumber of non-insulated and insulated strands 5. Around the conductor 4there is an inner semiconducting layer 6, which is in contact with atleast some of the non-insulated strands 5A. This semiconducting layer 6is in its turn surrounded by the main insulation of the cable in theform of an extruded solid insulating layer 7. The insulating layer issurrounded by an external semiconducting layer 8. The conductor area ofthe cable can vary between 80 and 3000 mm² and the external diameter ofthe cable between 20 and 250 mm.

FIG. 3 shows schematically an ungrounded electric high voltage ACmachine in the form of a Y-connected generator or motor 14 directlyconnected to a power system 16.

FIG. 4 shows grounding means in the form of an overvoltage protector,like a non-linear resistance arrester 18, connected between the neutralpoint 20 of the Y-connected machine 14 and ground. Such a non-linearresistance arrester 18 connected to the neutral point protects theinsulated conductor used in the machine windings against transientovervoltages, such as overvoltages caused by a stroke of lightning.

FIG. 5 shows an embodiment with a high ohmic resistor 22 connected inparallel to the non-linear resistance arrester 18. The non-linearresistance arrester 18 is functioning in the same way in this embodimentas in the embodiment shown in FIG. 4 and with the resistor 22 asensitive detection of ground faults by measuring the voltage across theresistor 22 is realised.

FIG. 6 shows an embodiment with high resistance grounding of the neutralpoint 20. In this embodiment a technique similar to the prior artdescribed in connection with FIG. 1 is used. Thus a resistor 24 isconnected to the secondary 26 of a transformer with the primary winding28 of the transformer connected from the neutral point 20 of the machine14 to ground. The resistor 24 is comparatively low ohmic and of ruggedconstruction, as compared to a high ohmic resistor which would be neededfor direct connection between the neutral point 20 and ground forobtaining the same result. The voltage class of the resistor canconsequently be reduced. Also in this case a non-linear resistancearrester 18 is connected in parallel to the primary winding 28. Withthis embodiment mechanical stresses and fault damages are limited duringline-to-ground faults by restricting the fault current. Transientovervoltages are limited to safe levels and the grounding device is moreeconomical than direct insertion of a resistor.

Resonant grounding of the machine can be realised in a similar way byreplacing the resistor 24 by a reactor having characteristics such thatthe capacitive current during a line-to-ground fault is neutralized byan equal component of inductive current contributed for by the reactor.Thus the net fault current is reduced by the parallel resonant circuitthus formed and the current will be essentially in phase with the faultvoltage. After extinction of the fault the voltage recovery on thefaulted phase will be very slow and determined by the ratio of inductivereactance to the effective resistance of the transformer/reactorcombination. Accordingly any ground fault of transient nature willautomatically be extinguished in such a resonant grounded system. Thussuch resonant grounding means limits the ground fault current topractically zero, thus minimising the mechanical stresses Furthercontinued operation of the machine can be permitted after the occurrenceof a phase-to-ground fault until an orderly shutdown can be arranged.

FIG. 7 shows an embodiment with a non-linear resistance arrester 18connected between the neutral point 20 and ground and a groundingtransformer 30 connected on the network side of the machine 14. Thegrounding transformer 30 is of Y-Δ design with the neutral point of theY-connection connected to ground, whereas the Δ-winding is isolated.Grounding transformers are normally used in systems which are ungroundedor which have a high impedance ground connection. As a system componentthe grounding transformer carries no load and does not affect the normalsystem behaviour. When unbalances occur the grounding transformerprovides a low impedance in the zero sequence network. The groundingtransformer is in this way equivalent to a low inductance or lowresistance grounding with respect to fault current levels and transientovervoltages.

The grounding transformer can also be a so-called zigzag transformerwith special winding arrangements, see e.g. Paul M. Anderson, “Analysisof Faulted Power Systems”, The Iowa State University Press/Ames, 1983,pp. 255-257.

Also a possible auxiliary power transformer can be used for suchgrounding purposes.

FIG. 8 shows an embodiment with a low ohmic resistor 32 connectedbetween the neutral point 20 of the machine 14 and ground. The mainadvantage of such a low resistance grounding is the ability to limittransient and temporary overvoltages. The currents will, however, behigher in case of single phase ground faults. Also third harmoniccurrents will be higher in undisturbed operation.

FIG. 9 shows an alternative embodiment of the machine according to theinvention in which the resistor 32 is replaced by a low inductanceinductor 34 connected between the neutral point 20 and ground. Lowinductance grounding works essentially in the same way as low ohmicgrounding. The value of the inductor 34 in ohms is less than thatrequired for resonant grounding, cf. description of FIG. 6.

As an alternative to the direct connection between the neutral point 20and ground of the resistor 32 or the inductor 34, they may be indirectlyconnected with the aid of a transformer whose primary is connectedbetween the neutral point 20 and ground and whose secondary contains theresistor or inductor, cf. the description of FIG. 6.

In FIG. 10 an embodiment is shown comprising two impedances 36 and 38connected in series between the neutral point 20 of the machine 14 andground, the impedance 36 having a low impedance value and the impedance38 a high impedance value. The impedance 38 can be short-circuited by ashort-circuiting device 40. In normal operation the short-circuitingdevice 40 is open in order to minimize third harmonic currents. In caseof a ground fault the short-circuiting device 40 is controlled toshort-circuit the impedance 38 and the potential in the neutral point 20will be low and the current to ground comparatively high.

In case of an internal ground fault in the machine 14 the impedance 38is not short-circuited. As a consequence the voltage will be high in theneutral point 20 but the current to ground will be limited. In such asituation this is to prefer since a high current can give rise todamages in this case.

To be able to cope with the problems arising from third harmonics whendirectly connecting an AC electric machine to a three-phase powernetwork, i.e. when no step-up transformer is used between the machineand the network, grounding means in the form of a suppression filter 35,37, tuned to the third harmonic together with an overvoltage protector39 can be used, see FIG. 11. The filter thus comprises a parallelresonance circuit consisting of an inductor 35 and a capacitivereactance 37. The dimensioning of the filter 35, 37 and its overvoltageprotector 39 is such that the parallel circuit is capable of absorbingthird harmonics from the machine 14 during normal operation, yetlimiting transient and temporary overvoltages. In case of a fault theovervoltage protector 39 will limit the fault voltage such that thefault current flows through the overvoltage protector 39 if the fault isconsiderable. In case of a single-phase ground fault the currents willbe higher as compared to e.g. the case of high resistance groundingsince the fundamental impedance is low.

In FIG. 12 an embodiment is shown wherein the grounding means comprisesa detuned switchable third harmonics depression filter connected inparallel to an overvoltage protector 40. Such filters can be realised inseveral different ways. FIG. 12 shows an example comprising twoinductors 42, 44 connected in series and a capacitor 46 connected inparallel to the series-connected inductors 42, 44. Further ashort-circuiting device 48 is connected across the inductor 44.

The short-circuiting device 48 is controllable to change thecharacteristic of the filter by short-circuiting the inductor 44 when arisk for third harmonic resonance between the filter and the machine 14and network 16 is detected. This is described more in detail in Swedishpatent application 9700347-9. In this way third harmonic currents arestrongly limited in normal operation. Transient and temporaryovervoltages will be limited and the currents will be higher in case ofa single-phase ground fault in the same way as described in connectionwith FIG. 11.

FIG. 13 shows an embodiment wherein the neutral point 20 of the machine14 is directly connected to ground, at 21. Such direct grounding limitstransient and temporary overvoltages but results in high currents incase of ground faults. Third harmonic current flow from the neutral 20of the machine to ground will be comparatively high in normal operation.

As a further alternative the grounding means of the machine according tothe invention can comprise an active circuit for providing a connectionof the neutral point to ground having desirable impedance properties.

In FIG. 14 a Δ-connected three-phase machine 50 is shown directlyconnected to the distribution or transmission network 16.

In such a situation a grounding transformer of the same kind as the oneused in the embodiment shown in FIG. 7 can be connected on the networkside of the machine 50.

As in the embodiment of FIG. 7 the grounding transformer can be aY-Δ-connected transformer with the neutral point of the Y-connectionground, or a so called zigzag grounding transformer, that is aZ-0-connected transformer with the Z grounded. The grounding transformerwill limit temporary overvoltages.

As in the embodiment of FIG. 7 a possible auxiliary power transformercan also be used for this purpose.

1. An electric high voltage AC machine for direct connection to adistribution or transmission network, said machine including at leastone flexible winding and having a neutral point and comprising at leastone current-carrying conductor comprising a plurality of insulatedstrands and at least one uninsulated strand and a magneticallypermeable, electric field confining covering surrounding the conductor;a first layer having semi-conducting properties surrounding theconductor and being in electrical contact therewith, a solid insulatinglayer surrounding said first layer, and an outer layer havingsemi-conducting properties surrounding said insulating layer, andgrounding means for connecting the neutral point of said winding toground.
 2. The machine according to claim 1, wherein the potential ofsaid first layer is substantially equal to the potential of theconductor.
 3. The machine according to claim 1, wherein the potential ofsaid first layer is substantially equal to the potential of theconductor.
 4. The machine according to claim 3, wherein said secondlayer is connected to a predetermined potential.
 5. The machineaccording to claim 4, wherein said predetermined potential is groundpotential.
 6. The machine according to claim 1, wherein at least twoadjacent layers have substantially equal thermal expansion coefficients.7. The machine according to claim 1, wherein said layers are adjacent toeach other, an each of said layers has at least one connecting surfaceeach being fixedly connected to the connecting surface of the adjacentlayer along substantially the whole of said connecting surface.
 8. Themachine according to claim 1, wherein said grounding means comprisemeans for low-resistance grounding of the winding.
 9. The machineaccording to claim 8, said machine having a Y-connected winding neutralpoint and wherein said low-resistance grounding means comprise alow-resistance resistor connected between the neutral point and ground.10. The machine according to claim 8, said machine having a Y-connectedwinding the neutral point further comprising a transformer having aprimary and a secondary winding and wherein said low-resistancegrounding means comprises a resistor connected in the secondary of thetransformer whose primary is connected between the neutral point andground.
 11. The machine according to claim 1, wherein said groundingmeans comprise means for low-inductance grounding of the winding. 12.The machine according to claim 11, said machine having a Y-connectedwinding the neutral point and wherein said low-inductance groundingmeans comprises a low-inductance inductor connected between the neutralpoint and ground.
 13. The machine according to claim 11, said machinehaving a Y-connected winding neutral point, further comprising atransformer having a primary and a secondary winding and wherein saidlow-inductance grounding means comprises an inductor connected in thesecondary of the transformer whose primary is connected between theneutral point and ground.
 14. The machine according to claim 1, whereinsaid grounding means comprise means for high-resistance grounding of thewinding.
 15. The machine according to claim 14, said machine having aY-connected winding neutral point and wherein said high-resistancegrounding means comprise a high-resistance resistor connected betweenthe neutral point and ground.
 16. The machine according to claim 14,said machine having a Y-connected winding neutral point furthercomprising a transformer having a primary and a secondary winding andwherein said high-resistance grounding means comprise a resistorconnected in the secondary of the transformer whose primary is connectedbetween the neutral point and ground.
 17. The machine according to claim1, wherein said grounding means comprise means for high-inductancegrounding of the winding.
 18. The machine according to claim 17, saidmachine having a Y-connected winding the neutral point and wherein saidhigh-inductance grounding means comprises a high-inductance inductorconnected between the neutral point and ground.
 19. The machineaccording to claim 17, said machine having a Y-connected winding neutralpoint further comprising a transformer having a primary and a secondarywinding and wherein said high-inductance grounding means comprises aninductor connected in the secondary of the transformer whose primary isconnected between the neutral point and ground.
 20. The machineaccording to claim 1, said machine having a Y-connected winding neutralpoint, further comprising a transformer having a primary and a secondarywinding and wherein said grounding means comprises a reactor connectedin the secondary of the transformer whose primary is connected betweenthe neutral point and ground, said reactor having characteristics suchthat capacitive current during a ground fault is substantiallyneutralized by an equal component of inductive current contributed forby the reactor.
 21. The machine according to claim 1, wherein saidgrounding means comprises means for changing the impedance of theconnection to ground in response to a ground fault.
 22. The machineaccording to claim 1, wherein said grounding means comprises an activecircuit.
 23. The machine according to claim 1, wherein said groundingmeans comprises a Y-Δgrounding transformer connected to the network sideof the machine.
 24. The machine according to claim 1, wherein saidgrounding means comprise a zigzag grounding transformer connected to thenetwork side of the machine.
 25. The machine according to claim 1, saidmachine having a Y-connected winding neutral point wherein saidgrounding means comprise a suppression filter tuned for the n:thharmonic.
 26. The machine according to claim 1, said machine having aY-connected winding neutral point wherein said grounding means comprisea switchable suppression filter detuned for the n:th harmonic.
 27. Themachine according to claim 25, wherein said n:th harmonic is the thirdharmonic.
 28. The machine according to claim 1, said machine having aY-connected winding neutral point wherein said grounding means comprisean overvoltage protector connected between said neutral point andground.
 29. The machine according to claim 1, said machine having aY-connected winding neutral point wherein an overvoltage protector isconnected between said neutral point and ground in parallel to saidgrounding means.
 30. A distribution or transmission network, whichcomprises at least one machine according to claim
 1. 31. An electric ACmachine having a magnetic circuit for high voltage comprising: amagnetic core and at least one winding, wherein said winding is formedof a flexible cable comprising at least one current-carrying conductorand a magnetically permeable, electric field confining coveringsurrounding the conductor, each conductor having a number of insulatedconductor elements and at least one uninsulated conductor element, andinner semi-conducting layer surrounding the conductor and being inelectric contact with at least one of the conductor elements, aninsulating layer of solid insulating material surrounding said innersemi-conducting layer, and an outer semi-conducting layer surroundingsaid insulating layer, and grounding means for connection to at leastone selected point of said winding to ground.
 32. The machine accordingto claim 31, wherein said grounding means comprise means for directgrounding of the winding.
 33. A high voltage electric machine comprisingat least one winding, wherein said winding comprises a flexible cableincluding at least one current-carrying conductor comprising a pluralityof insulated strands and at least one uninsulated strand, and amagnetically permeable, electric field confining covering surroundingthe conductor including an inner semi-conducting layer surrounding theconductor and being in electrical contact therewith, a solid insulatinglayer surrounding the inner layer, and an outer semi-conducting layersurrounding the insulating layer, said inner and outer layers formingequipotential surfaces around the conductor, said cable forming at leastone uninterrupted turn in the corresponding winding of said machine. 34.The machine of claim 33, wherein the cover is formed of a plurality oflayers including an insulating layer and wherein said plurality oflayers are substantially void free.
 35. The machine of claim 33, whereinthe cover is an electrical contact with the conductor.
 36. The machineof claim 35, wherein the layers of the cover have substantially the sametemperature coefficient of expansion.
 37. The machine of claim 33,wherein the cover is heat resistant such that the machine is operable of100% overload for two hours.
 38. The machine of claim 33, wherein themachine, when energized, produces an electric field and the coverconfines the electric field so that the cable is operable free ofsensible end winding loss.
 39. The machine of claim 33, wherein themachine, when energized, produces an electric field and the coverconfined the electric field so that the winding is operable free ofpartial discharge and field control.
 40. The machine of claim 33,wherein the winding comprises multiple uninterrupted turns.
 41. Themachine of claim 33, wherein the cable comprises a transmission line.